Kinetic lability

Rates of Reaction. The rates of formation and dissociation of displacement reactions are important in the practical appHcations of chelation. Complexation of many metal ions, particulady the divalent ones, is almost instantaneous, but reaction rates of many higher valence ions are slow enough to measure by ordinary kinetic techniques. Rates with some ions, notably Cr(III) and Co (III), maybe very slow. Systems that equiUbrate rapidly are termed kinetically labile, and those that are slow are called kinetically inert. Inertness may give the appearance of stabiUty, but a complex that is apparentiy stable because of kinetic inertness maybe unstable in the thermodynamic equihbrium sense. 

These are thermodynamic constants which relate to the system when it has reached equilibrium, and must be distinguished from any considerations of kinetic lability or inertness which refer to the speed with which that equilibrium is attained. 

A further factor which must also be taken into consideration from the point of view of the analytical applications of complexes and of complex-formation reactions is the rate of reaction to be analytically useful it is usually required that the reaction be rapid. An important classification of complexes is based upon the rate at which they undergo substitution reactions, and leads to the two groups of labile and inert complexes. The term labile complex is applied to those cases where nucleophilic substitution is complete within the time required for mixing the reagents. Thus, for example, when excess of aqueous ammonia is added to an aqueous solution of copper(II) sulphate, the change in colour from pale to deep blue is instantaneous the rapid replacement of water molecules by ammonia indicates that the Cu(II) ion forms kinetically labile complexes. The term inert is applied to those complexes which undergo slow substitution reactions, i.e. reactions with half-times of the order of hours or even days at room temperature. Thus the Cr(III) ion forms kinetically inert complexes, so that the replacement of water molecules coordinated to Cr(III) by other ligands is a very slow process at room temperature. 

The trans-effect is, therefore, a kinetic labilizing effect rather than a thermodynamic one. An approximate series is ... 

So, the active M—C bond in the propagation centers of heterogeneous catalysts is quite stable, its short lifetime being determined by its high kinetic lability as a result of the possibility of various reactions proceeding in the coordination sphere of transition metals. 

There is a large amount of data available concerning the thermodynamic effects of ligands on other coordination sites (i. e., the thermodynamic cis- and iraws-effects). However, very little is known about the effects of ligands on the kinetic lability of other coordination sites. In fact, very little work has been carried out, directly with Bi2-derivatives, or with models of B12, on the kinetics of ligand substitution at the cobalt center. Of particular biochemical interest would be studies on the rate of displacement of coordinated benzimidazole by various ligands. Such work has not been reported at present. If the benzimidazole is replaced during enzymatic catalysis so that the lower axial position is occupied by some other Lewis base, one would expect this displacement, and the reverse step, to be very facile. This appears to be qualitatively true in that when water displaces benzimidazole as the benzimidazole is pro-... 

Ludwick and Brown (137) have investigated the kinetic lability of various ligands (L) in the system CH3Co(dimethylglyoximato)2L, dmg-dimethylglyoximato anion. Unlike Bi2-derivates, attempts to remove a... 

Goshe, A.J., Steele, I.M., Ceccarelli, C., Rheingold, A.L. and Bosnich, B. (2002) Supramolecular chemistry and self-assembly special feature supramolecular recognition on the kinetic lability of thermodynamically stable host-guest association complexes. Proceedings of the... 

Applications involving ring transfer or loss. The kinetic lability, volatility, and Lewis acidity of heavy alkaline earth metallocenes have been the properties most important to their applications. The gas-phase decomposition of volatile metallocenes is useful in the preparation of thin films of alkaline earth-containing materials and in doping semiconductors. Reviews are available on the use of group 2 organometallic compounds as precursors for chemical-vapour deposition (CVD).2 3... 

Cr2+ (d4) are kinetically labile, as are high-spin complexes of Co2+ (d7). However, complexes of Cr3+ (d3) and low-spin complexes of Co3+ (d6) are kinetically inert. For the exchange reaction (O represents lsO)... 

In line with expectations of kinetic inertness for third-row transition metals, little interest has been vested in the development of osmium anticancer drugs, as ligand-exchange rates did not seem favorable on the timescale of cellular processes. Our work, however, shows that the kinetic lability of such complexes can be timed to such extent that anticancer activity comes within range. We have demonstrated how rational chemical design can thus be applied to osmium-arene complexes resulting in specific... 

The need to achieve high yield in one-pot synthesis, coupled to the relative kinetic inertness of rhenium complex (e.g. compared to technetium) and the mild conditions required has led to the development of useful versatile rhenium(V) intermediates that can be quickly prepared in quantitative yield, and are metastable, i.e. kinetically labile enough to react rapidly with the final chelator, again in high yield. The most widely used ligands suitable for this purpose are polydentate hydroxycarboxylic acids such as glucoheptonate [116a], citrate (47), tartrate (48), and 2-hydroxyisobutyric acid (49) [159]. Examples are discussed elsewhere in this chapter. They are typically used in the presence of Sn(II) to reduce Re(VII) to Re(V), at moderately elevated temperature (50-100 °C) at pH 2-3 (acid pH promotes reduction of perrhenate, presumably by facilitat-... 

In spite of the above mentioned Co(EII) compounds, kinetically labile metal complexes may provide fast product/substrate exchange and some of these systems show real catalytic activity. In native dinuclear phosphatases Mg(II), Mn(II), Fe(II/III), or Zn(II) ions are present in the active centers. Although the aqua complexes of the weakest Lewis acids, Mg(H) and Mn(II), show measurable acceleration of e.g. the transesterification of 2-hydroxypropyl p-nitrophenyl phosphate HPNP, [Mn(II)] = 0.004 M, kobs/ uncat = 73 at pH 7 and 310 K, [38] or the hydrolysis of S -uridyluridine (UpU) [39], only a few structural [40] but no functional phosphatase-mimicking dinuclear complexes have been reported with these metal ions. 

The dimer complex 13 showed, for the first time for kinetically labile complexes, an enhanced activity toward the hydrolysis of the activated... 

In order to destabilize the likely unproductive 6-membered chelate structure of type K (Scheme 17) that might be formed if the catalyst reacts with the 4-pen-tenoate entity, the cyclization was run in the presence of a Lewis acid which competes with the evolving carbene for the Lewis basic ester group. Such an additive has to be compatible with the RCM catalyst, should provoke a minimum of acid-catalyzed side reactions, and must undergo a kinetically labile coordination with the relay substituent. Ti(OiPr)4 was found to meet these stringent requirements... 

---